It is known that to reduce tire and road wear, it is advantageous to provide steering mechanisms for multiple axle trailers. One example of such a mechanism is shown in U.S. Pat. No. 5,207,443. This is an example of a true four wheel steering mechanism in which each of four wheels on the trailer is independently steerable (i.e. pivotable relative to its associated axle) and which therefore allows optimum steering geometry to be selected. Other more primitive mechanisms exist, in which the wheels on only one axle are independently steerable, but these systems whilst offering a reduced turning circle still suffer from tire wear on the wheels which are not independently steerable and consequently give rise to road damage.
The mechanism disclosed in U.S. Pat. No. 5,207,443 steers the wheels using a linkage mechanism for each pair of wheels, operated by relative movement of a sub-chassis on which the wheels are mounted and a draw bar which is articulated to a tractor unit. As the tractor unit rounds a corner, the draw bar follows it, but the sub-chassis initially does not. Thus, the draw bar moves relative to the sub-chassis, activating the steering allowing the trailer to follow the tractor unit. This mechanism, although highly satisfactory in achieving four wheel steering, is somewhat complicated. This stems from the need to be able to reverse the direction in which the trailer wheels steer depending upon whether the trailer is being drawn or backed up by a tractor unit. To achieve this, a hydraulically activated device is used for each linkage mechanism, i.e. for each pair of steerable wheels.
The present invention is designed to address the problems outlined above and to provide a simplified reversible steering mechanism for a trailer having four independently steerable wheels.